Flame-retardant layers which are translucent, i.e. light transmissive, have been provided heretofore and can be generated from hydrated alkali metal silicate compositions which can contain auxiliary agents and are heated in a drying atmosphere and dried to the solid layer. Such layers also can be considered to consist of expandable material because of a tendency toward bubble formation therein.
The auxiliary materials which can be utilized in such aqueous compositions include urea, polyhydric alcohols, monosaccharides, polysaccharides, sodium phosphate, sodium aluminate, aluminum phosphate, borax, boric acid and colloidal silica.
Such layers have been provided in the form of self-supporting plates or slabs, and also can be used as layers between glass panes so that the laminated structure, i.e. a sandwich construction of the panes and the flame-resistant layer, can constitute a fire-barrier wall or a portion of a fire barrier.
The solid flame resistant layer can also be provided as a foil, e.g. upon a glass pane serving as a substrate and can remain thereon when the assembly is built into a fire-barrier wall, or can be removed from the substrate to be utilized as a flame-retardant foil as desired. The drying is usually carried out in a drying chamber.
In the method described in German patent document (open application) DE-OS No. 28 33 718, the drying atmosphere is ambient air. The flame resistant layers formed by this method have been found to manifest, as early as the drying stage and as well thereafter, various changes which can be considered as "aging" and which appear to be the effect of light and heat and especially ultraviolet radiation. Microbubbles and larger bubbles in the expanded product appear to be generated in a form which interferes with the optical properties of the layer and even with the most careful drying, crack formation is observed.
To minimize these effects, the aforementioned auxiliary changes have been admixed with the hydrated alkali metal silicate (see the aforementioned German patent publication). Nonetheless aging-dependent bubble formations and deterioration of the optical properties is observed.